Historically, radios were realized by incorporating various discrete components into a system such that radio frequency (RF) signals received at an antenna could be processed to output audio signals of a given band. Such discrete circuitry however is consuming in ways of cost and space. Furthermore, typically the circuitry is dedicated to a particular frequency band and thus a multi-band radio could require much circuitry.
More recently, semiconductor-based radios have been introduced that provide for multi-band operation. Even in these radios, certain amounts of duplicative circuitry may be needed to handle the multiple bands. In addition, coupling an antenna to a semiconductor-based radio can create some challenges. For example in design of a receiver, the input voltage received is sought to be maximized so that the input signal-to-noise ratio (SNR) is maximized. However, impedance of an antenna will vary significantly across frequency, especially across multiple frequency bands. Furthermore, the antenna impedance is often unknown and depends heavily on the environment. For example, the impedance of a headphone antenna that typically is used in portable audio devices can be quite different depending on the distance of the portable device to the human body.
Therefore, one design philosophy is to form an external parallel resonance network so that the impedance of the RF front-end network including a RF low noise amplifier (LNA) appears as high impedance to the antenna. By doing that, the received voltage from the antenna will be received at the RF LNA input with minimal loss. Varactors are typically used to form the parallel resonance network in order to resonate across frequency. However, it is difficult if not possible to resonate a single varactor across multiple frequency bands. For example, a resonating range from 76 MHz to 240 MHz is needed in order to cover both the FM/HD (76-108 MHz) and DAB/DAB+/T_DMB VHF band (174-240 MHz). To do that, the maximum/minimum capacitance ratio of the varactor needs to be over 10, which can be very difficult to achieve in current semiconductor technologies using a single varactor and inductor.